Filters are used to remove contaminants from fluid streams. In various applications, filters made from fibrous structures are used to remove dust particles from air streams. With conventional filters made from round cross-section fibers, as the dust is collected by the fibrous structures, there is a build-up of pressure drop with an increase in the dust retained by the filters. Filters are generally replaced once the pressure-drop across the filters reaches a pre-determined value. Particularly, filters made from submicron size fibers, such as melt blown fibers, have a very rapid build up of pressure with increased dust retention. This results in significantly reduced filter life. Grooved fibers provide unique capability to trap and store dust particles in the fiber grooves from the fluid stream that are smaller than the groove size and can therefore hold large amounts of dust without any significant rise in pressure drop. Thus, it is shown in the present invention that by utilizing nonwoven structures made from grooved fibers (as a first layer) in series with melt blown fibrous structures, the useful life of the composite filter structure can be significantly enhanced without adversely affecting the pressure drop. For particles as small as 0.1.mu. NaCl, high filtration efficiencies are obtained at pressure drops essentially the same as in conventional fiber filters. In principle, one can make composite filter structures, wherein the second layer can be one having higher filtration efficiency than the first layer, but a greater pressure-drop than the first layer. The first layer is made from grooved fibers. The first layer may additionally contain binder fibers.
The fibrous structures may be electrostatically charged also. There may be a third layer to support the first two layers. When we have a composite structure, a large percentage of contaminant is removed by the first layer containing the grooved fibers. The first layer has a low pressure drop and a very large "dust capacity" as compared to the second layer. The second layer has a high filter efficiency. The composite fibrous structure has enhanced filter performance--a significant improvement in filter life.
U.S. Pat. No. 5,336,286 discloses a high efficiency air filtration media composed of fibrids, binder fiber, and staple fiber.
U.S. Pat. No. 4,904,174 discloses an apparatus for electrically charging melt blown webs.
U.S. Pat. No. 4,215,682 discloses melt blown fibrous electrets which improve the filtration efficiency of the filter media, however, the melt blown structures have a relatively high pressure drop. U.S. Pat. No. 4,714,647 discloses melt blown filter material having different fiber sizes along the depth of the filter. Japanese Patent 6198108A discloses composite filtering material made from sheets of inorganic fiber (glass fiber) and sheets of synthetic (organic) fiber.
Also, the following are of interest:
U.S. Pat. No. 3,490,211 PA1 Article from October 1994 issue of Chemical Engineering--pp 181, 119, 120 PA1 1994 "Nonwovens Conference" Proceedings, pp 9-11 PA1 Air Filtration Improved by Corona Charging of Fibrous Materials--Wadsworth and Tsai, Textiles and Nonwovens Development Center (TANDEC), University of Tennessee, Knoxville, Tenn. PA1 European Patent Application 0 558 091 A1 PA1 U.S. Pat. No. 4,824,451.